Apparatus and method for sequentially resetting elements of a magnetic sensor array

ABSTRACT

A semiconductor process and apparatus provide a high-performance magnetic field sensor with three differential sensor configurations which require only two distinct pinning axes, where each differential sensor is formed from a Wheatstone bridge structure with four unshielded magnetic tunnel junction sensor arrays, each of which includes a magnetic field pulse generator for selectively applying a field pulse to stabilize or restore the easy axis magnetization of the sense layers to orient the magnetization in the correct configuration prior to measurements of small magnetic fields. The field pulse is sequentially applied to groups of the sense layers of the Wheatstone bridge structures, thereby allowing for a higher current pulse or larger sensor array size for maximal signal to noise ratio.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. patent application Ser. No.13/031,558 filed Feb. 21, 2011.

TECHNICAL FIELD

The exemplary embodiments described herein generally relate to the fieldof magnetoelectronic devices and more particularly to CMOS compatiblemagnetoelectronic field sensors used to sense magnetic fields.

BACKGROUND

Sensors are widely used in modern systems to measure or detect physicalparameters, such as position, motion, force, acceleration, temperature,pressure, etc. While a variety of different sensor types exist formeasuring these and other parameters, they all suffer from variouslimitations. For example, inexpensive low field sensors, such as thoseused in an electronic compass and other similar magnetic sensingapplications, generally are anisotropic magnetoresistance (AMR) baseddevices. In order to arrive at the required sensitivity and reasonableresistances that mesh well with CMOS, the sensing units of such sensorsare generally on the order of square millimeters in size. For mobileapplications, such AMR sensor configurations are too costly, in terms ofexpense, circuit area, and power consumption.

Other types of sensors, such as magnetic tunnel junction (MTJ) sensorsand giant magnetoresistance (GMR) sensors, have been used to providesmaller profile sensors, but such sensors have their own concerns, suchas inadequate sensitivity and being effected by temperature changes. Toaddress these concerns, MTJ, GMR, and AMR sensors have been employed ina Wheatstone bridge structure to increase sensitivity and to eliminatetemperature dependent resistance changes. For minimal sensor size andcost, MTJ or GMR elements are preferred. Typically, a Wheatstone bridgestructure uses magnetic shields to suppress the response of referenceelements within the bridge so that only the sense elements (and hencethe bridge) respond in a predetermined manner. However, the magneticshields are thick and their fabrication requires carefully tuned NiFeseed and plating steps. Another drawback associated with magneticshields arises when the shield retains a remnant field when exposed to astrong (˜5 kOe) magnetic field, since this remnant field can impair thelow field measuring capabilities of the bridge structure. To prevent theuse of magnetic shields, a Wheatstone bridge structure may include twoopposite anti-ferromagnetic pinning directions for each sense axis,resulting in four different pinning directions which must beindividually set for each wafer, very often requiring complex andunwieldy magnetization techniques. There are additional challengesassociated with using MTJ sensors to sense the earth's magnetic field,such as accounting for variations in the measured field caused byBarkhausen noise, sporadic depinning, and jumps of micro-magneticdomains as the sense element responds to an applied field. Priorsolutions have attempted to address these challenges by pinning the endsof the sense element in the MTJ sensor, either through a hard magneticbias layer or an anti-ferromagnetic pinning layer, or by applying afield along the easy axis of the sense element during measurement. Thesesolutions add processing cost/complexity and/or consume additional powerduring measurement.

Accordingly, it is desirable to provide a magnetoelectronic sensor andmethod that is adaptable for measuring various physical parameters.There is also a need for a simple, rugged and reliable sensor that canbe efficiently and inexpensively constructed as an integrated circuitstructure for use in mobile applications. There is also a need for animproved magnetic field sensor and method to overcome the problems inthe art, such as outlined above. Furthermore, other desirable featuresand characteristics of the exemplary embodiments will become apparentfrom the subsequent detailed description and the appended claims, takenin conjunction with the accompanying drawings and the foregoingtechnical field and background.

BRIEF SUMMARY

A field sensor is configured for resetting sense elements prior tomeasurement of a field. The field sensor comprises a first bridgecircuit including a first plurality of magnetic elements configured tosense a field in a first dimension, the first plurality including igroups of magnetic elements where i is greater than or equal to 2; andcircuitry configured to apply a current pulse sequentially to andadjacent to each of the i groups and thereby configuring the fieldsensor for measurement of the field in the first dimension

In yet another field sensor, a first bridge circuit includes a firstplurality of magnetic elements configured to sense a field in a firstdirection, the first plurality including at least a first and secondgroup of magnetic elements; a first conductive line positioned adjacentthe first group; a second conductive line positioned adjacent the secondgroup; a second bridge circuit including a second plurality of magneticelements configured to sense the field in a second direction, the secondplurality including at least a third and fourth group of magneticelements; a third conductive line positioned adjacent the third group; afourth conductive line positioned adjacent the fourth group; a thirdbridge circuit including a third plurality of magnetic elementsconfigured to sense a field in a third direction, the third pluralityincluding at least a fifth and sixth group of magnetic elements; a fifthconductive line positioned adjacent the fifth group; a sixth conductiveline positioned adjacent the sixth group; and circuitry configured toapply a current pulse sequentially to each of the first through sixthconductive lines and thereby configuring the field sensor formeasurement of the components of the field in the first, second, andthird directions.

In a method of resetting a field sensor, applying a first current pulseto a first reset line positioned adjacent each of a first group ofmagnetic elements within a first bridge circuit; and subsequentlyapplying a second current pulse to a second reset line positionedadjacent each of a second group of magnetic elements within the firstbridge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 illustrates two active sense elements having magnetizations thatare angled equally in different directions from a pinned layer that willdeflect in response to an externally applied magnetic field and providean output signal related to the component of the magnetic field that isnot aligned with the pinning direction of the pinned layer;

FIG. 2 illustrates an electronic compass structure which usesdifferential sensors formed from two bridge structures with unshieldedMTJ sensors, along with the circuit output for each bridge structure;

FIG. 3 is a simplified schematic perspective view of a Wheatstone bridgecircuit in which series-connected MTJ sensors are aligned to havedifferent magnetization directions from the magnetization direction ofthe pinned layer;

FIG. 4 is a partial schematic perspective view of first and second MTJsensors which include a magnetic field generator structure for clearingor stabilizing the sense layer prior to or during sense operations;

FIG. 5 is a partial cross-sectional view of an integrated circuit inwhich the first and second MTJ sensors shown in FIG. 4 are formed tohave sense layers with different magnetization directions;

FIG. 6 is an example plot of the magneto-resistance against the appliedfield when no stabilization field is applied to the sensor;

FIG. 7 is an example plot of the magneto-resistance against the appliedfield when a steady state stabilization field is applied to the sensor;

FIG. 8 is an example plot of the magneto-resistance against the appliedfield when a pre-measurement reset pulse is applied to the sensor;

FIG. 9 is a simplified schematic top or plan view of a reticle layoutshowing differential sensor formed with a plurality of series-connectedMTJ sensors configured in a Wheatstone bridge circuit with a magneticfield generator structure positioned in relation to the MTJ sensors;

FIG. 10 is a partial block diagram of three Wheatstone bridgesconfigured to receive sequential application of reset pulses forresetting a plurality of MTJ sensors within each bridge;

FIG. 11 is a partial block diagram of another three Wheatstone bridgesconfigured to receive sequential resetting pulses for resetting aplurality of MTJ sensors within each bridge and sensing a magneticfield;

FIG. 12 is a schematic diagram of the circuitry for generating thesequential resetting pulses; and

FIG. 13 is a flowchart showing a method of sequentially resetting theMTJ field sensors which may be used to provide initialize magnetic fieldsensing elements.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

A method and apparatus are described for a differential sensor array inwhich sense elements formed over pinned layers are dynamicallystabilized with an aligning field pulse that is sequentially appliedperiodically (e.g., during each measurement cycle) to groups of thedifferential sensor array. Using shape anisotropy, the shapes of twosense elements arrays may be formed to have magnetizations that areangled equally in different directions from a single magnetizationdirection of the pinned layer so that the sense layers will deflect inresponse to an externally applied magnetic field. With thisconfiguration, a single axis magnetic sensor may be formed from a singlepinning direction, or a compass circuit may be formed from three suchdifferential sensors circuits so that only two pinning directions arerequired for the entire three axis compass, thereby simplifying andreducing the manufacturing cost and complexity. In an exampleimplementation, each differential sensor circuit is constructed as aWheatstone bridge structure in which four arrays of unshielded activesense elements are used to detect and measure an externally appliedmagnetic field. To address field fluctuations that can impair the fieldresponse of a sense element, the sensor layers may be dynamicallystabilized by applying a field pulse either before each fieldmeasurement or at a predetermined interval to prepare the magneticsensor, thereby eliminating the need for any hard bias layer(s) tostabilize the sense elements. Without such hard bias stabilization or ameasurement preparation capability, a momentary exposure to a largefield may reorient the magnetization of the sense elements in a poorlydetermined state. The sense array is optimally sized for highestpossible signal-to-noise (SNR) while allowing the available voltagesupply to stabilize the sensors with the required current during themeasurement phase. For the smallest possible physical array size, andhence the lowest cost, a single copper line (or parallel connectedseries of line segments) is routed underneath each sense element. In thestabilization (or measurement) phase, each of these line segments isconnected in series so that each sense element has the identicalstabilization field and hence identical sensor response. Therefore, thearray averaging is precise and SNR is increased the maximal amount for agiven array size. To prepare the sense elements for field measurement,an orienting field pulse is applied along the stabilization path. Forthis operation, the stabilization line is broken into groups of segmentsthat may be connected sequentially to the available voltage source,wherein isolated combinations of the groups of segments may beoptionally reset in parallel. The orienting current pulse train thenproceeds sequentially down a reset/stabilization line that is placedproximal to the sense elements and the groups of sense elements areprepared for measurement. As all the groups in each sense axis must beproperly configured (oriented) for measurement of that axis, thisorientation procedure must be applied at some point between a high fieldexposure and before each axis proceeds to the measurement phase. Allsense axes may be reset together, or a reset may be applied before eachindividual axis is measured. This reset may be periodic, precede eachmeasurement, or only occur when an error condition (very high bridgeoffset indicating misorientation, linearity errors, or high noisecondition) is encountered. As the individual sense element anisotropy islarge compared with the stabilization field that must be applied fornoise optimization but on the same order as the field required toreorient the sense element, the magnitude of the orientation field pulseis much large than that applied during the measurement phase for sensorstabilization. Therefore, the available voltage is insufficient to resetan array that is sized for maximal SNR. In order to have sufficientvoltage, the preparation phase described herein is a sequential seriesof orientation pulses, wherein each pulse treats a different subsectionof the array. The pulse duration is very short, preferably 20 ns orless, and hence in this manner all sense elements across all bridges maybe prepared for measurement in an extremely short time window.Subsequent to the preparation phase, all line segments within a bridgeor sense axis are connected in series, and stabilization current isapplied to these segments and a measurement may proceed.

Various illustrative embodiments of the present invention will now bedescribed in detail with reference to the accompanying figures. Whilevarious details are set forth in the following description, it will beappreciated that the present invention may be practiced without thesespecific details, and that numerous implementation-specific decisionsmay be made to the invention described herein to achieve the devicedesigner's specific goals, such as compliance with process technology ordesign-related constraints, which will vary from one implementation toanother. While such a development effort might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure. Inaddition, selected aspects are depicted with reference to simplifiedcross sectional drawings without including every device feature orgeometry in order to avoid limiting or obscuring the present invention.It is also noted that, throughout this detailed description,conventional techniques and features related to magnetoresistive RandomAccess Memory (MRAM) design, MRAM operation, semiconductor devicefabrication, and other aspects of the integrated circuit devices may notbe described in detail herein. While certain materials will be formedand removed to fabricate the integrated circuit sensors as part of anexisting MRAM fabrication process, the specific procedures for formingor removing such materials are not detailed below since such details arewell known and not considered necessary to teach one skilled in the artof how to make or use the present invention. Furthermore, thecircuit/component layouts and configurations shown in the variousfigures contained herein are intended to represent exemplary embodimentsof the invention. It should be noted that many alternative or additionalcircuit/component layouts may be present in a practical embodiment.

It will be appreciated that additional processing steps will be used tofabricate of MTJ sensor structures. As examples, one or more dielectric,ferromagnetic and/or conductive layers may be deposited, patterned andetched using well known techniques, along with conventional backendprocessing (not depicted), typically including formation of multiplelevels of interconnect that are used to connect the sensor structures ina desired manner to achieve the desired functionality. Thus, thespecific sequence of steps used to complete the fabrication of thesensor structures may vary, depending on the process and/or designrequirements.

The disclosed fabrication process may be used to form a magnetic fieldsensor from two differential sensor configurations for a single axisresponse, with only a single pinned direction. For two axis (X,Y)magnetic field response, the sensor only requires two distinct pinningaxes, where each differential sensor is formed from a bridge structureswith unshielded magnetic tunnel junction (MTJ) sensors. For a third axis(Z), no additional pinning direction is required. The distinct pinningaxes may be obtained using shape anisotropy of differently shapedpinning layers in combination with a carefully selected anneal process,or by forming two distinct pinning layers which are separately set andannealed. In a given differential sensor formed from MTJ sensorsconnected in a bridge circuit, shape anisotropy may be used to createsense elements having different magnetizations at zero field that areangled at different orientations from the magnetization of the pinnedlayer, for example, at negative 45 degrees and positive 45 degrees. Inthis configuration, an applied field that includes a component that isorthogonal to the pinning direction will change the magnetization of thedifferent sense layers differently, and as a result, the differentialsensor is able to measure the projection of the applied fieldperpendicular to the pinned axis. The disclosed fabrication process alsoforms a field conductor below and optionally above each MTJ sensor thatmay be used to apply a field pulse along the easy axis of the senselayers to prepare the sensor for measurement, and a smaller current tostabilize the sensor during measurement if desired.

Turning now to FIG. 1, a sensor structure 1 is shown in simplifiedschematic form which uses two active sense element types 20, 30 and apinned layer 10 to measure an external magnetic field. As depicted, themagnetization directions 21, 31 of the active sense elements 20, 30 areangled equally and in different directions from the magnetizationdirection of a pinned layer 10. To this end, the sense elements 20, 30may be formed so that the shape of each sense element is elongated(i.e., longer) in the direction of the desired magnetization for thatsense element. Thus shaped, the sense elements 20, 30 use their shapeanisotropy to create magnetization directions that are offset from thepinned layer 10. For example, the first sense element 20 may be formedso that its preferred magnetization direction is angled at −45 degreesfrom the magnetization direction of the pinned layer 10, and with thesecond sense element 30 so that its preferred magnetization direction isangled at 45 degrees from the magnetization direction of the pinnedlayer 10, although other offset angles may be used.

Because the resistance/conductance across a sense element and pinnedlayer depends on the cosine of the angle between the sense element andthe pinned layer, the resistance/conductance of the sensor structure canbe changed by applying an external magnetic field (H) which deflects themagnetization of the sensor elements 20, 30. For example, if there is noapplied field (H=0) to a sensor structure 1, then the magnetizationdirections 21, 31 of the sense elements 20, 30 are unchanged, and thereis no difference between the resistance/conductance of the first andsecond sensor elements 20, 30. And if an external field H is applied toa sensor structure 2 that is directed along or anti-parallel to thepinned layer 10, the applied field will deflect or rotate the magneticmoments 22, 32 of the sensor elements 20, 30 equally, resulting in equalresistance/conductance changes for each sense element, and hence nochange in their difference. However, when an external field H is appliedto a sensor structure 3 that is orthogonal to the pinned layer 10, themagnetic moments 23, 33 for each sense element 20, 30 are changeddifferently in response to the applied field. For example, when theexternal field H shown in FIG. 1 is directed to the right, theresistance/conductance of the first sense element 20 is reduced, whilethe resistance/conductance of the second sense element 30 is increased,resulting in a difference signal that is related to the field strength.In this way, the depicted sensor structure measures the projection ofthe applied field perpendicular to the pinned axis, but not parallel toit.

FIG. 2 shows first and second sensors 201, 211 for detecting thecomponent directions of an applied field along a first y-axis (Axis 1)and a second x-axis (Axis 2), respectively. As depicted, each sensor isformed with unshielded sense elements that are connected in a bridgeconfiguration. Thus, the first sensor 201 is formed from the connectionof sense elements 202-205 in a bridge configuration over a pinned layer206 that is magnetized in a first direction. In similar fashion, thesecond sensor 211 is formed from the connection of sense elements212-215 in a bridge configuration over a pinned layer 216 that ismagnetized in a second direction that is perpendicular to themagnetization direction of the pinned layer 206. In the depicted bridgeconfiguration 201, the sense elements 202, 204 are formed to have afirst magnetization direction and the sense elements 203, 205 are formedto have a second magnetization direction, where the first and secondmagnetization directions are orthogonal with respect to one another andare oriented to differ equally from the magnetization direction of thepinned layer 206. As for the second bridge configuration 211, the senseelements 212, 214 have a first magnetization direction that isorthogonal to the second magnetization direction for the sense elements213, 215 so that the first and second magnetization directions areoriented to differ equally from the magnetization direction of thepinned layer 216. In the depicted sensors 201, 211, there is noshielding required for the sense elements, nor are any special referenceelements required. In an example embodiment, this is achieved byreferencing each active sense element (e.g., 202, 204) with anotheractive sense element (e.g., 203, 205) using shape anisotropy techniquesto establish the easy magnetic axes of the referenced sense elements tobe deflected from each other by 90 degrees.

By positioning the first and second sensors 201, 211 to be orthogonallyaligned with the orthogonal sense element orientations in each sensorbeing deflected equally from the sensor's pinning direction, the sensorscan detect the component directions of an applied field along the firstand second axes. This is illustrated in FIG. 2 with the depicted circuitsimulation shown below each sensor. In each simulation, the simulatedbridge output 207, 217 is a function of an applied field angle for senseelements with an anisotropy field of 10 Oe, applied field of 0.5 Oe, anda magnetoresistance of 100% when the sense element switches from ananti-parallel state to a parallel state. The simulated bridge outputscan be used to uniquely identify any orientation of the applied externalfield. For example, a field that is applied with a 0 degree field angle(e.g., pointing “up” so that it is aligned with the y-axis or Axis 1)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of 10 mV/V from the second sensor 201.Conversely, a field that is applied in the opposite direction (e.g.,pointing “down” so that it is aligned with a 180 degree field angle)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of −10 mV/V from the second sensor 201.

As seen from the foregoing, a magnetic field sensor may be formed fromdifferential sensors 201, 211 which use unshielded sense elements202-205, 212-215 connected in a bridge configuration over respectivepinned layers 206, 216 to detect the presence and direction of anapplied magnetic field. With this configuration, the possibility ofresidual magnetic moment present in magnetic shielding is eliminated. Inaddition, the magnetic field sensor provides good sensitivity, and alsoprovides the temperature compensating properties of a bridgeconfiguration. By eliminating the need to form magnetic shieldinglayers, the manufacturing complexity and cost is reduced and the size ofthe sensor structure is decreased (in terms of eliminating the siliconreal estate required to form any shielding layers). There are alsoperformance benefits to using unshielded sense elements since themagnetic remnance problem is eliminated by removing the magneticshielding layers.

FIG. 3 provides a simplified schematic perspective view of an examplefield sensor 300 formed by connecting four MTJ sensors 301, 311, 321,331 in a Wheatstone bridge circuit, where the series-connected MTJsensors 301, 311, 321, 331 are formed with sense layers 302, 312, 322,332 that are aligned to have different magnetization directions from themagnetization direction of the pinned layers 304, 314, 324, 334. Thedepicted sensor 300 is formed with MTJ sensors 301, 311, 321, 331 thatmay be manufactured as part of an existing MRAM manufacturing processwith only minor adjustments to control the orientation of the magneticfield directions for different layers. In particular, each MTJ sensor301, 311, 321, 331 includes a first pinned electrode 304, 314, 324, 334,an insulating tunneling dielectric layer 303, 313, 323, 333, and asecond sense electrode 302, 312, 322, 332. The pinned and senseelectrodes are desirably magnetic materials, for example, and notintended to be limiting, NiFe, CoFe, Fe, CoFeB and the like, or moregenerally, materials whose magnetization can be collectively aligned.Examples of suitable electrode materials and arrangements are thematerials and structures commonly used for electrodes ofmagnetoresistive random access memory (MRAM) devices, which are wellknown in the art and contain, among other things, ferromagneticmaterials. The pinned and sense electrodes may be formed to havedifferent coercive force or field requirements. The coercive field isbasically the amount of field that is required to reverse the magnetfrom one direction to another after saturation. Technically, it is themagnetic field required to return the magnetization of the ferromagnetto zero after it has been saturated. For example, the pinned electrodes304, 314, 324, 334 may be formed with an anti-ferromagnetic filmexchange coupled to a ferromagnetic film to with a high coercive fieldso that their magnetization orientation can be pinned so as to besubstantially unaffected by movement of an externally applied magneticfield. In contrast, the sense electrodes 302, 312, 322, 332 may beformed with a magnetically soft material to provide different anisotropyaxes having a comparatively low coercive force so that the magnetizationorientation of the sense electrode (in whatever direction it is aligned)may be altered by movement of an externally applied magnetic field. Inselected embodiments, the coercive field for the pinned electrodes isabout two orders of magnitude larger than that of sense electrodes,although different ratios may be used by adjusting the respectivecoercive fields of the electrodes using well known techniques to varytheir composition and/or pinning strength.

As shown in FIG. 3, the pinned electrodes 304, 314, 324, 334 in the MTJsensors are formed to have a first exemplary anisotropy axis alignmentin the plane of the pinned electrode layers 304, 314, 324, 334(identified by the vector arrows pointing toward the top of the drawingof FIG. 3). As described herein, the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 may be obtained using shapeanisotropy of the pinned electrodes, in which case the shapes of thepinned electrodes 304, 314, 324, 334 would each be longer in thedirection of the “up” vector arrow for a single layer pinned magneticstack. In addition or in the alternative, the anisotropy axis alignmentfor the pinned electrodes 304, 314, 324, 334 may be obtained by formingone or more magnetic layers in the presence of a saturating magneticfield that is subsequently or concurrently annealed and then cooled sothat the magnetic field direction of the pinned electrode layers is setin the direction of the saturating magnetic field. As will beappreciated, the formation of the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 must be reconciled with thefabrication steps used to form any other field sensors which includepinned electrodes having a distinct anisotropy axis alignment, as wellas any fabrication steps used to form any sense electrodes having adistinct anisotropy axis alignment.

The depicted field sensor 300 also includes MTJ sensors 301, 321 inwhich sense electrodes 302, 322 are formed to have an exemplaryanisotropy axis (identified by the left-pointing vector arrows) that isoffset from the anisotropy axis of the pinned electrodes by a firstdeflection angle. In addition, the depicted field sensor 300 includesMTJ sensors 311, 331 in which sense electrodes 312, 332 are formed tohave an exemplary anisotropy axis (identified by the right-pointingvector arrows) that is offset from the anisotropy axis of the pinnedelectrodes by a second deflection angle which is equal but opposite tothe first deflection angle. In a particular embodiment, the firstdeflection angle is perpendicular to the second deflection angle so thatanisotropy axis of the sense electrodes 302, 322 is rotated negative 45degrees with respect to the anisotropy axis of the pinned electrodes,and so that anisotropy axis of the sense electrodes 312, 332 is rotatedpositive 45 degrees with respect to the anisotropy axis of the pinnedelectrodes.

As will be appreciated, the MTJ sensors 301, 311, 321, 331 may be formedto have identical structures that are connected as shown in series bymetal interconnections in a standard Wheatstone bridge circuitconfiguration with both power supply terminals 341, 343 and outputsignal terminals 342, 344 for the bridge circuit being shown. Byconnecting in series the unshielded MTJ sensors 301, 311, 321, 331 in aWheatstone bridge circuit, the field sensor 300 detects the horizontaldirection (left to right in FIG. 3) component of an externally appliedmagnetic field, thereby forming an X-axis sensor bridge. In particular,a horizontal field component would deflect the magnetization of thesense electrodes 302, 322 differently from the deflection of themagnetization of the sense electrodes 312, 332, and the resultingdifference in sensor conductance/resistance would quantify the strengthof the horizontal field component. Though not shown, a Y-axis sensorbridge circuit may also be formed with unshielded MTJ sensors connectedin a Wheatstone bridge circuit configuration, though the anisotropy axisof the pinned electrodes in the Y-axis sensor bridge circuit would beperpendicular to the anisotropy axis of the pinned electrodes 304, 314,324, 334 in the X-axis sensor bridge. Each of these sensors (or bridgelegs) 301, 311, 321, 331 may represent an array of sense elementsworking in concert to increase the overall SNR of the system.

Low field magnetic sensors are susceptible to Barkhausen noise, sporadicde-pinning, jumps of micro-magnetic domains (resulting from differentregions in the magnetic sense element that may have slightly differentorientations of their local magnetic moment from in weak local pinningthat is caused by edge roughness caused by small local inhomogeneitiesin the sense layer), or a myriad of other sources. Such noise canintroduce errors in accurately measuring the angular resolution of theEarth's magnetic field. When a field is applied, these micro-magneticdomains may reverse in a sequential fashion in lieu of the desiredcoherent rotation of the sense element. Prior attempts to address suchnoise have used a hard magnetic bias layer in the sense layers to pinthe ends of the device. However, hard bias layers can reduce thesensitivity of the sensor, and have the additional disadvantages ofrequiring an additional processing layer, etch step and anneal step.

To address the Barkhausen noise problem, a magnetic field may beselectively applied along the easy axis of the sense element prior toperforming a measurement. In selected embodiments, the magnetic field isapplied as a brief field pulse that is sufficient to restore themagnetic state of the sense element and remove micro-magnetic domainsthat may have appeared as the result of exposure to a strong field. Inan example implementation, a field pulse is applied to a sensor toremove metastable pinned regions in the sense element, where the fieldpulse has a threshold field strength (e.g., above approximately 40 Oe)and a minimum pulse duration (e.g., of approximately 20-100nanoseconds). By applying such a field pulse with a predeterminedmeasurement period (e.g., 10 Hz) as required for a compass application,the resulting field pulse has an extremely low duty cycle and minimalpower consumption. In addition, by terminating the field pulse prior tomeasurement, there is no additional field applied to the sense elementduring measurement, resulting in maximal sensitivity.

To illustrate an example of how a field pulse may be applied to a senseelement, reference is now made to FIG. 4, which shows a partialschematic perspective view of first and second MTJ sensors 410, 420which each include a magnetic field generator structure 414, 424 forresetting or stabilizing the sense layer 411, 421 prior to or duringsense operations. Each MTJ sensor may be constructed as shown in FIG. 4where the magnetic direction of the sense layer determines theorientation of the magnetic field generator structure. In particular,each MTJ sensor generally includes an upper ferromagnetic layer 411,421, a lower ferromagnetic layer 413, 423, and a tunnel barrier layer412, 422 between the two ferromagnetic layers. In this example, theupper ferromagnetic layer 411, 421 may be formed to a thickness in therange 10 to 10000 Angstroms, and in selected embodiments in the range 10to 100 Angstroms, and functions as a sense layer or free magnetic layerbecause the direction of its magnetization can be deflected by thepresence of an external applied field, such as the Earth's magneticfield. As for the lower ferromagnetic layer 413, 423, it may be formedto a thickness in the range 10 to 2000 Angstroms, and in selectedembodiments in the range 10 to 100 Angstroms, and functions as a fixedor pinned magnetic layer when the direction of its magnetization ispinned in one direction that does not change magnetic orientationdirection during normal operating conditions. As described above, thefirst and second MTJ sensors 410, 420 may be used to construct adifferential sensor by forming the lower pinned layers 413, 423 to havethe same magnetization direction (not shown), and by forming themagnetization direction 415 in upper sense layer 411 to be orthogonal tothe magnetization direction 425 in upper sense layer 421 so that themagnetization directions 415, 425 are oriented in equal and oppositedirections from the magnetization direction of the lower pinned layers413, 423.

To restore the original magnetization of the upper sense layers 411, 421that can be distorted by magnetic domain structure, FIG. 4 depicts amagnetic field generator structure 414, 424 formed below each sensor. Inselected embodiments, the magnetic field generator structure 414, 424 isformed as a conducting current line which is oriented to create amagnetic field pulse which aligns with the magnetization direction 415,425 in the upper sense layer 411, 421. For example, when a current pulseflows through the magnetic field generator structure 414 below the firstMTJ sensor 410 in the direction indicated by the arrow 416, a fieldpulse is created that is aligned with the easy axis 415 of the senseelement 411 in the first MTJ sensor 410. However, since the second MTJsensor 420 has a sense layer 421 with a different magnetizationdirection 425, the magnetic field generator structure 424 is oriented sothat a field pulse is created that is aligned with the easy axis 425 ofthe sense element 421 in the second MTJ sensor 420 when a current pulseflows through the magnetic field generator structure 424 in thedirection indicated by the arrow 426.

The exemplary embodiments described herein may be fabricated using knownlithographic processes as follows. The fabrication of integratedcircuits, microelectronic devices, micro electro mechanical devices,microfluidic devices, and photonic devices involves the creation ofseveral layers of materials that interact in some fashion. One or moreof these layers may be patterned so various regions of the layer havedifferent electrical or other characteristics, which may beinterconnected within the layer or to other layers to create electricalcomponents and circuits. These regions may be created by selectivelyintroducing or removing various materials. The patterns that define suchregions are often created by lithographic processes. For example, alayer of photo resist material is applied onto a layer overlying a wafersubstrate. A photo mask (containing clear and opaque areas) is used toselectively expose this photo resist material by a form of radiation,such as ultraviolet light, electrons, or x-rays. Either the photo resistmaterial exposed to the radiation, or that not exposed to the radiation,is removed by the application of a developer. An etch may then beapplied to the layer not protected by the remaining resist, and when theresist is removed, the layer overlying the substrate is patterned.Alternatively, an additive process could also be used, e.g., building astructure using the photo resist as a template.

Various illustrative embodiments of the process integration will now bedescribed in detail with reference to the accompanying figures. Whilevarious details are set forth in the following description, it will beappreciated that the present invention may be practiced without thesespecific details, and that numerous implementation-specific decisionsmay be made to the invention described herein to achieve the devicedesigner's specific goals, such as compliance with process technology ordesign-related constraints, which will vary from one implementation toanother. While such a development effort might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure. Inaddition, selected aspects are depicted with reference to simplifiedcross sectional drawings without including every device feature orgeometry in order to avoid limiting or obscuring the present invention.It is also noted that, throughout this detailed description,conventional techniques and features related to magnetic sensor designand operation, Magnetoresistive Random Access Memory (MRAM) design, MRAMoperation, semiconductor device fabrication, and other aspects of theintegrated circuit devices may not be described in detail herein. Whilecertain materials will be formed and removed to fabricate the integratedcircuit sensors as part of an existing MRAM fabrication process, thespecific procedures for forming or removing such materials are notdetailed below since such details are well known and not considerednecessary to teach one skilled in the art of how to make or use thepresent invention. Furthermore, the circuit/component layouts andconfigurations shown in the various figures contained herein areintended to represent exemplary embodiments of the invention. It shouldbe noted that many alternative or additional circuit/component layoutsmay be present in a practical embodiment.

The relative alignment of the field pulse and easy axis directions mayalso be seen in FIG. 5, which depicts a partial cross-sectional view ofan integrated circuit device in which the first and second MTJ sensorsshown in FIG. 4 are formed to have sense layers 411, 421 with differentmagnetization directions. In particular, the cross-sectional view on theleft shows the first MTJ sensor 410 as seen from the perspective view 5Ain FIG. 4, while the cross-sectional view on the right shows the secondMTJ sensor 420 as seen from the perspective view 5B in FIG. 4. The firstand second MTJ sensors 410, 420 are each formed over a substrate 430,440 which may have an active circuit 431, 441 embedded therein. On thesubstrate, one or more circuit layers 432, 442 may be formed beforeforming an insulating layer 433, 443 in which a conductive line 414, 424is embedded to form a magnetic field generator structure. As shown inFIG. 5, the conductive line 414 in the first MTJ sensor 410 is formed tocarry current in the direction coming out of plane of the drawing ofFIG. 5, while the conductive line 424 in the second MTJ sensor 420 isformed to carry current moving right-to-left on the drawing. Over theembedded conductive lines, the first and second MTJ cores are formed inan insulating layer 435, 445. In particular, the first MTJ core in thefirst MTJ sensor 410 includes a first conductive line 434 at leastpartially embedded in the insulating layer 435, a lower pinnedferromagnetic layer 413, a tunnel barrier layer 412, an upper senseferromagnetic layer 411 having a magnetization direction 415 that isoriented right-to-left, and a second conductive line 436 over which isformed an additional dielectric layer 437. The first conductive layer434 is connected to a bottom contact layer 438 through a via structure439. In addition, the second MTJ core in the second MTJ sensor 420includes a first conductive line 444 at least partially embedded in theinsulating layer 445, a lower pinned ferromagnetic layer 423, a tunnelbarrier layer 422, an upper sense ferromagnetic layer 421 having amagnetization direction 425 that is oriented into the plane of thedrawing of FIG. 5, and a second conductive line 446 over which is formedan additional dielectric layer 447. To connect the first and second MTJsensors 410, 420, the first conductive layer 444 in the second MTJsensor 420 is connected through a via structure (not shown) to a bottomcontact layer (not shown) in the same level as the embedded conductiveline 424, which in turn is connected through one or more vias andconductive layers to the second conductive line 436 from the first MTJsensor 410. With the depicted configuration, current pulses through theembedded conductive line 414 will create a magnetic field pulse 417which is aligned with the easy axis 415 of the sense element 411, andcurrent pulses through the embedded conductive line 424 will create amagnetic field pulse in the region of the sense element 421 (not shown)which is aligned with the easy axis 425 of the sense element 421.

The lower pinned ferromagnetic layers 413, 423 may be a material, forexample, iridium manganese, platinum manganese, cobalt iron, cobalt ironboron, nickel iron, ruthenium, and the like, or any combination thereof.The tunnel barrier layers 412, 422 may be a material, for example,aluminum oxide or magnesium oxide. The upper sense ferromagnetic layers411, 421 may be a material, for example, nickel iron, cobalt iron,cobalt iron boron, ruthenium, and/or the like. The magnetic fieldgenerator structures 414, 424 may be aluminum, copper, tantalum,tantalum nitride, titanium, titanium nitride or the like, whileconductive lines in general may be, for example, aluminum, copper,tantalum, tantalum nitride, titanium, titanium nitride or the like.

The first and second MTJ sensors 410, 420 may be fabricated together ona monolithic integrated circuit as part of a differential sensor byforming sense layers 411, 421 having orthogonal magnetic orientationsthat each differ equally from the magnetic direction of the pinnedlayers 413, 423. In an example process flow, the first step in thefabrication process is to provide a monolithic integrated circuit chipsubstrate which is covered by a dielectric base layer (not shown). Overthe dielectric base layer, magnetic field generator structures 414, 424are formed as embedded lines of conductive material using knowndeposition, patterning and etching processes so that the magnetic fieldgenerator structures 414, 424 are aligned and positioned below thesensors 410, 420 and embedded in an insulating layer (not shown). Uponthe insulating layer, a stack of sensor layers is sequentially formed bydepositing a first conductive layer (to serve after etching as theconductive line 434), one or more lower ferromagnetic layers (to serveafter etching as the lower pinned ferromagnetic layer 413), one or moredielectric layers (to serve after etching as the tunnel barrier layer412), one or more upper ferromagnetic layers (to serve after etching asthe upper sense ferromagnetic layer 411), and a second conductive layer(to serve after etching as the conductive line 436).

While the various ferromagnetic layers may each be deposited and heatedin the presence of a magnetic field to induce a desired magneticorientation, shape anisotropy techniques may also be used to achieve therequired magnetic orientations for the different ferromagnetic layers.To this end, the sensor layer stack is selectively etched with asequence of patterned etch processes to define the pinned and senselayers in the MTJ sensors 410, 420. In a first etch sequence, the shapesof the different pinning layers 413, 423 are defined from the lowerferromagnetic layer(s) by using patterned photoresist to form a firstpatterned hard mask and then performing a selective etch process (e.g.,reactive ion etching) to remove all unmasked layers down to andincluding the unmasked lower ferromagnetic layer(s). The resultingshapes of the etched lower ferromagnetic layers are oriented so thateach pinned layer has shape anisotropy, resulting in a preferredmagnetic orientation along one of its axes. In addition to being formedas long and narrow shapes, additional shaping of the ends of pinnedlayers may be provided so that each of the pinned layers performs morelike a single magnetic domain. For example, the pinned layers 901, 902,903, 904 shown in FIG. 9 layers may be shaped to have pointed ends thattaper in the corresponding directions of desired pinned direction forthe pinned layers. Using shape anisotropy, the shaped pinned layers 413,423 may be annealed to set their respective pinning directions.

At this point in the fabrication process, the upper ferromagneticlayer(s) will have been selectively etched to leave a remnant portionunder the first patterned hard mask so that the upper and lowerferromagnetic layer(s) have the same shape. However, the final shape ofthe sense layers will be smaller than the underlying pinned layers, andto this end, a second etch sequence is used to define the final shapesof the different sense layers 411, 421 from the remnant portions of theupper ferromagnetic layer(s). In the second etch sequence, anotherphotoresist pattern is used to form a patterned hard mask over the partsof the remnant upper ferromagnetic layer(s) layer that will form thesense layers. The pattern is selected to define high aspect ratio shapesfor the sense layers when a selective etch process (e.g., reactive ionetching) is used to remove all unmasked layers down to and including theunmasked upper ferromagnetic layer(s) 411, 421. In selected embodiments,the selective etch process may leave intact the underlying shaped pinnedlayers 413, 423, though in other embodiments, the selective etch processalso etches the unmasked portions of the underlying shaped pinned layers413, 423. The defined high aspect ratio shapes for the sense layers areoriented so that the sense layers 411 are longer in the dimension of thedesired magnetization 415 than they are wide, while the sense layers 421are longer in the dimension of the desired magnetization 425 than theyare wide. In other words, the long axis for each sense layer is drawnalong the desired magnetization direction for a single ferromagneticsense layer. In addition to being formed as long and narrow shapes,additional shaping of the ends of sense layers 411, 421 may be providedso that each of the sense layers performs more like a single magneticdomain. For example, the sense layers may be shaped to have pointed endsthat taper in the corresponding directions of desired easy axis for thesense layers. Once the shaped sense layers are formed, the desired easyaxis magnetic orientations may be induced from their shape anisotropy bybriefly annealing the wafer (e.g., at an anneal temperature ofapproximately 250 degrees C. in the absence of a magnetic field toremove material dispersions. Upon cooling, the magnetizations of thesense layers 411, 421 align with the individual pattern, providingmultiple orientations of sense layers.

By controlling the magnitude and timing of the current flow through themagnetic field generator structures 414, 424 so as to create a fieldpulse just prior to using the sensors 410, 420 to perform a fieldmeasurement, the sense layers 411, 421 are prepared before eachmeasurement in a way that maintains high sensitivity and minimizes powerconsumption. The benefits of selectively applying a magnetic field tothe sense elements are demonstrated in FIGS. 6-8. Starting with FIG. 6,there is provided an example plot of the magneto-resistance against theapplied field when no stabilization field is applied to the sensor.Without a stabilization field, the micro-magnetic domain jumps cause thetransfer curve 60 to have sporadic, unpredictable jumps inmagneto-resistance (a.k.a., Barkhausen noise) as an applied field isswept. This noise may be prevented by applying a weak stabilizationfield in alignment with the easy axis of the sense element. For example,FIG. 7 provides an example plot of the magneto-resistance against theapplied field when a 15 Oe easy axis stabilization field is applied as asteady state field to the sensor. As shown in the plot of FIG. 7, themicro-magnetic domain jumps have been eliminated. As a consequence, thetransfer curve 70 in this example includes a region of linearcharacteristics 71 up to an applied field of approximately 20 Oe. Inaddition or in the alternative, a field pulse may be applied to furtherimprove the transfer curve, as shown in FIG. 8 which provides an exampleplot 80 of the magneto-resistance against the applied field when apulsed stabilization field is applied to the sensor. In particular, thetransfer curve 80 was obtained by briefly pulsing a sensor element alongits easy axis immediately prior to performing a field measurement at thesensor with a sequence of field sweeps, starting with a first sweep from−5 Oe to 5 Oe, and then performing a second sweep from −10 Oe to 10 Oe,and so on. The resulting transfer curve 80 includes a region of linearcharacteristics 81 up to at least an applied field of approximately 20Oe. In addition, the transfer curve 80 indicates that poor performancemay be created when this sensor is exposed to a hard axis field aboveapproximately 40 Oe. Stated more generally, a strong field applied at anarbitrary direction may put the sense element in a bad state, while afield pulse applied along the sensor easy axis is sufficient to removedomain structure from the sense element.

In a practical deployment, the magnetic field generator structures 414,424 are formed from the same layer that is necessary to interconnect thebridge legs, and hence creates no additional processing steps. Inaddition, each of the magnetic field generator structures 414, 424 maybe constructed from a single conductive element that is positioned topass beneath each MTJ sensor with the appropriate orientation, therebycreating field pulses throughout the chip with a single current pulse.An example of such a practical implementation is illustrated with FIG. 9which provides a simplified schematic top or plan view of a reticlelayout showing differential sensor 900 formed with a plurality ofseries-connected MTJ sensors 921, 922, 923, 924 configured in aWheatstone bridge circuit with a magnetic field generator structure 920positioned in relation to the MTJ sensors. The depicted differentialsensor includes four pinned layers 901, 902, 903, 904 which each havethe same magnetization direction (e.g., a pinned axis in they-direction), as shown by the large vector arrow on each pinned layer.While the pinned layers 901, 902, 903, 904 may be formed using theirshape anisotropy (as indicated in FIG. 9), they may also be formed usinga traditional field-anneal process.

FIG. 9 also shows that two of the MTJ sensors or sensor arrays, 921, 924in the differential sensor are formed with sense layers 911, 914 havinga magnetization direction that is oriented at 45 degrees from vertical,as shown with the easy axis vector pointing to the right in the senselayers 911, 914. The other two MTJ sensors 902, 903 are formed withsense layers 912, 913 having a magnetization direction that is orientedat negative 45 degrees from vertical, as shown with the easy axis vectorpointing to the left in the sense layers 912, 913. While any desiredtechnique may be used to form the sense layers having differentmagnetization directions, selected embodiments of the present inventionuse shape anisotropy techniques to shape the sense elements 911, 914 tohave a magnetization direction (or easy axis) that is oriented atpredetermined deflection angle from vertical, and to shape the senseelements 912, 913 to have a magnetization direction (or easy axis) thatis oriented negatively at the predetermined deflection angle fromvertical. In this way, the magnetization direction of the sense elements911, 914 and the magnetization direction of the sense elements 912, 913are offset equally in opposite directions from the magnetizationdirection of the pinned layers 901, 902, 903, 904.

The depicted differential sensor 900 also includes a magnetic fieldgenerator structure 920 which is formed beneath the MTJ sensors 921,922, 923, 924 so as to selectively generate a magnetic field tostabilize or restore the magnetic field of the sense layers 911, 912,913, 914. In selected embodiments, the magnetic field generatorstructure 920 is formed as a single conductive line which is arranged tocarry current beneath the sense layers 911, 912, 913, 914 in a directionthat is perpendicular to the easy axis orientation of the sense layersso that the magnetic field created by the current is aligned with theeasy axis. Thus, the conductive line 920 is formed below the fourth MTJsensor 924 to create a magnetic field that is aligned with the easy axisof the sense element 914. In addition, the orientation of the conductiveline 920 below the second and third MTJ sensors 922, 923 creates amagnetic field that is aligned with the easy axis of the sense elements912, 913. Finally, the conductive line 920 is formed below the first MTJsensor 921 to create a magnetic field that is aligned with the easy axisof the sense element 911.

Referring to FIG. 10, a block diagram of an integrated magnetic sensor1002 includes the Wheatstone bridge 1004 for sensing a magnetic field inan X direction, a Wheatstone bridge 1006 for sensing a magnetic field ina Y direction, and a Wheatstone bridge 1008 for sensing a magnetic fieldin a Z direction. While three Wheatstone bridges 1004, 1006, 1008 areshown for sensing a field in three dimensions, it is understood thatonly one Wheatstone bridge may be formed for sensing a field in onedimension, or two Wheatstone bridges may be formed for sensing a fieldin two dimensions. One example of a structure for sensing a magneticfield in three dimensions may be found in U.S. patent application Ser.No. 12/567,496. The Wheatstone bridge 1004 includes a first plurality ofmagnetic elements 1014, including a first group 1024 and a second group1034. Likewise, Wheatstone bridge 1006 includes a first plurality ofmagnetic elements 1016, including a first group 1026 and a second group1036, and Wheatstone bridge 1008 includes a first plurality of magneticelements 1018, including a first group 1028 and a second group 1038.While two groups are shown in each of the Wheatstone bridges 1004, 1006,1008, any number of groups numbering two or more may be utilized. Forexample, the plurality of magnetic elements 1014 may comprise i magneticelements, the plurality of magnetic elements 1016 may comprise jmagnetic elements, and the plurality of magnetic elements 1018 maycomprise k magnetic elements. Each of the magnetic elements 1020 withineach of the plurality of magnetic elements 1014, 1016, 1018 comprises amagnetic element such as MTJ sensors 410, 420 of FIGS. 4 and 5. A writedriver 1025 is configured to provide a current pulse to each of the MTJelements within the group 1024 and a write driver 1035 is configured tosubsequently provide a current pulse to each of the MTJ elements withinthe group 1034. Likewise, a write driver 1027 is configured to provide acurrent pulse to each of the MTJ elements within the group 1026 and awrite driver 1037 is configured to provide a current pulse to each ofthe MTJ elements within the group 1036, and a write driver 1029 isconfigured to provide a sequential current pulse to each of the MTJelements within the group 1028 and a write driver 1039 is configured toprovide a sequential current pulse to each of the MTJ elements withinthe group 1038.

Logic circuit 1041 sequentially selects write drivers 1025, 1035, 1027,1037, 1029, 1039, for sequentially providing a reset current pulse tothe groups 1024, 1034, 1026, 1036, 1028, 1038, respectively. Since agiven voltage is provided, by dividing a reset current line having alarge resistance into several reset current line segments each having asmaller resistance, a larger current may be applied to each group of MTJelements in the groups 1024, 1034, 1026, 1036, 1028, 1038. Also, thereset pulses will be sequential, and a smaller stabilization currentwill be applied during the measurement. This stabilization current willflow through the entire line and stabilize all sense elements with anidentical stabilization field since and the voltage is high enough tosupply the required stabilization line over the full line resistance.This embodiment is preferred as power consumption is lowest and smalldifferences in stabilization line resistances will not result indifferent stabilization fields. However, for higher SNR ratios than areattainable given this stabilization configuration, stabilization currentmay be applied to different sub groupings of the array in parallel,allowing the possibility of additional stabilization paths (and hencesense element array size increases) within a given sensor.

The logic circuit 1041 is further described in FIG. 11 and includes astate decoder 1102, a reset circuit 1104, and a read bridge selectcircuit 1106. Input signals 1111 and 1112 dictate one of four outputsignals 1113, 1114, 1115, 1116. Output signal 1113 activates the resetcircuit 1104 for sequentially activating each of the drivers 1025, 1035,1027, 1037, 1029, 1039. Output signals 1114, 1115, 1116 activate a readsequence of each of the Wheatstone bridges 1004, 1006, 1008.

Another magnetic sensor 1202 is shown in FIG. 12 and includes, as in themagnetic sensor 1002 of FIG. 10, the Wheatstone bridge 1004 for sensinga magnetic field in an X direction, a Wheatstone bridge 1006 for sensinga magnetic field in a Y direction, and a Wheatstone bridge 1008 forsensing a magnetic field in a Z direction. The Wheatstone bridge 1004includes a first plurality of magnetic elements 1014, including a firstgroup 1024 and a second group 1034. Likewise, Wheatstone bridge 1006includes a first plurality of magnetic elements 1016, including a firstgroup 1026 and a second group 1036, and Wheatstone bridge 1008 includesa first plurality of magnetic elements 1018, including a first group1028 and a second group 1034. While two groups are shown in each of theWheatstone bridges 1004, 1006, 1008, any number of groups numbering oneor more may be utilized.

A plurality of pads 1225, 1235, 1227, 1237, 1229, 1239 are coupled forproviding the current pulses to each of the groups 1024, 1034, 1026,1036, 1028, 1038, of magnetic elements, respectively. A logic and drivercircuit 1241, positioned off-chip for example, provides current pulsesto the pads 1225, 1235, 1227, 1237, 1229, 1239 sequentially, in asimilar manner to the structure of FIG. 9.

Operation of the magnetic sensors 1002, 1202 described herein may alsobe illustrated with reference to FIG. 13, which depicts an exemplaryflowchart for a method of operation of magnetic field sensors which donot exhibit micro-magnetic structure by resetting the magnetic elementsprior to or just after sensing of an external magnetic field. While twobridge circuits are described, it is understood than any number ofbridge circuits may be utilized. This resetting, or initializing,includes the steps of applying 1302 a first current pulse to a firstreset line positioned adjacent each of a first group of magneticelements within a first bridge circuit, applying 1304 a second currentpulse to a second reset line positioned adjacent each of a second groupof magnetic elements within the first bridge circuit, applying 1306 athird current pulse to a third reset line positioned adjacent each of athird group of magnetic elements within a second bridge circuit, andapplying 1308 a fourth current pulse to a fourth reset line positionedadjacent each of a fourth group of magnetic elements within the secondbridge circuit. A stabilization current is applied 1310 to thestabilization line. The magnetic elements are then accessed 1312 tomeasure a value of an external field projected along each of the sensoraxes of the magnetic elements.

By now it should be appreciated that there has been provided a magneticfield sensor apparatus and a method of operating a plurality ofdifferential sensor circuits over a substrate which detects an appliedmagnetic field directed along one or more axis. The differential sensorcircuits may be constructed as Wheatstone bridge structures, one foreach axis to be sensed, of unshielded magnetic tunnel junction (MTJ)sensors formed with a plurality of pinned layers that are eachmagnetized in a single pinned direction and a corresponding plurality ofunshielded sense layers. In an example implementation, the differentialsensor circuit includes a first unshielded MTJ sensor having a firstunshielded sense layer with a first easy axis magnetic orientation, anda second unshielded MTJ sensor having a second unshielded sense layerwith a second easy axis magnetic orientation, where the first and secondeasy axis magnetic orientations are deflected equally and in oppositedirections (e.g., +45 degrees) from the single pinned direction. Wheneach unshielded sense layer is formed to have an anisotropic axis with alonger length dimension and a shorter width dimension, the longer lengthdimension is aligned with an easy axis magnetic orientation for theunshielded sense layer. Each magnetic field sensor includes an embeddedmagnetic field generator disposed near each unshielded sense layer thatis positioned to generate a magnetic field pulse that is aligned with aneasy axis magnetic orientation for each unshielded sense layer. Inselected embodiments, the embedded magnetic field generator isimplemented as a conductive line positioned to conduct a current pulsethat creates a magnetic field pulse for resetting a magnetic orientationof an associated unshielded sense layer, and/or to apply a weak magneticfield along an easy axis magnetic orientation for each unshielded senselayer. The sense layers and conductive lines within each magnetic fieldsensor are grouped, wherein each group sequentially receives a currentpulse. By providing a current pulse sequentially to these groups, theline resistance during the orienting current pulse is reduced, allowingfor a larger current with the given voltage.

Although the described exemplary embodiments disclosed herein aredirected to various sensor structures and methods for making same, thepresent invention is not necessarily limited to the example embodimentswhich illustrate inventive aspects of the present invention that areapplicable to a wide variety of semiconductor processes and/or devices.Thus, the particular embodiments disclosed above are illustrative onlyand should not be taken as limitations upon the present invention, asthe invention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. For example, the relative positions of the sense andpinning layers in a sensor structure may be reversed so that the pinninglayer is on top and the sense layer is below. Also the sense layers andthe pinning layers may be formed with different materials than thosedisclosed. Moreover, the thickness of the described layers may vary.Accordingly, the foregoing description is not intended to limit theinvention to the particular form set forth, but on the contrary, isintended to cover such alternatives, modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims so that those skilled in the art shouldunderstand that they can make various changes, substitutions andalterations without departing from the spirit and scope of the inventionin its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

What is claimed is:
 1. A magnetic field sensor comprising: a firstbridge circuit including a first plurality of magnetic sense elementsconfigured to sense a magnetic field in a first dimension, the firstplurality of magnetic sense elements including i groups of magneticsense elements where i is greater than or equal to 2, and wherein eachgroup of the i groups of magnetic sense elements includes a plurality ofmagnetic sense elements to sense the magnetic field in the firstdimension; a second bridge circuit including a second plurality ofmagnetic sense elements configured to sense the magnetic field in asecond dimension, the second plurality including j groups of magneticsense elements where j is greater than or equal to 2, and wherein eachgroup of the j groups of magnetic sense elements includes a plurality ofmagnetic sense elements to sense the magnetic field in the seconddimension; and circuitry configured to generate a first plurality ofcurrent pulses and a second plurality of current pulses, the firstplurality of current pulses sequentially applied adjacent to each of thei groups of the plurality of magnetic sense elements and thereafter thesecond plurality of current pulses sequentially applied adjacent to eachof the j groups of the plurality of magnetic sense elements, wherein thesequential application of current pulses adjacent to the i and the jgroups of magnetic sense elements results in a larger current appliedadjacent to each of the i and j groups of magnetic sense elements, for agiven voltage, than if the current pulses had been applied adjacent tothe plurality of magnetic sense elements simultaneously.
 2. The magneticfield sensor of claim 1 wherein the circuitry further comprises: a logiccircuit; current driver circuitry configured to sequentially generateand apply the current pulses adjacent to the i groups of magnetic senseelements in response a control signal from the logic circuit.
 3. Themagnetic field sensor of claim 2 wherein the current driver circuitry isfurther configured to sequentially generate and apply the current pulsesadjacent to the j groups of magnetic sense elements in response acontrol signal from the logic circuit.
 4. The magnetic field sensor ofclaim 3 further including: one or more contact pads configured toreceive the current pulses; wherein the first and second bridge circuitsare located on a first chip and current driver circuitry is located on asecond chip.
 5. The magnetic field sensor of claim 1 further comprising:a third bridge circuit including a third plurality of magnetic senseelements configured to sense the magnetic field in a third dimension,the third plurality of magnetic sense elements including k groups ofmagnetic sense elements where k is greater than or equal to 2, whereineach group of the k groups of magnetic sense elements includes aplurality of magnetic sense elements to sense the magnetic field in thethird dimension, wherein the circuitry is configured to generate a thirdplurality of current pulses and sequentially apply the third pluralityof current pulses adjacent each of the k groups wherein the sequentialapplication of the current pulses to each of the k groups results in alarger current applied adjacent to each of the k groups, for a givenvoltage, than if the current pulse had been applied adjacent to all ofthe third plurality of magnetic sense elements simultaneously.
 6. Themagnetic field sensor of claim 5 wherein the circuitry comprises: alogic circuit; current driver circuitry, in response to control signalsfrom the logic circuit, configured to sequentially generate the currentpulses and sequentially apply the current pulses adjacent to the igroups of magnetic sense elements, j groups of magnetic sense elementsand k groups of magnetic sense elements.
 7. The magnetic field sensor ofclaim 6 further including: one or more a contact pads configured toreceive the current pulses; wherein the first, second and third bridgecircuits are located on a first chip and the logic circuit is located ona second chip.
 8. The magnetic field sensor of claim 5 wherein themagnetic sense elements of the first and second plurality of magneticsense elements each further comprise: a magnetic tunnel junction sensorhaving a first unshielded sense layer with an easy axis magneticorientation corresponding to the dimension associated with plurality ofmagnetic sense elements of the bridge circuit.
 9. The magnetic fieldsensor of claim 1 wherein each current pulse of the plurality of currentpulses includes a pulse width of 50 nanoseconds or less.
 10. A method ofresetting magnetic sense elements of a magnetic field sensor,comprising: sequentially resetting group of magnetic sense elements of afirst bridge circuit by sequentially applying current pulses to a firstplurality of reset lines, wherein each reset line of the first pluralityof reset lines is located adjacent an associated group of magnetic senseelements of a first bridge circuit, wherein each magnetic sense elementof the group of magnetic sense elements of the first bridge circuitsenses the magnetic field in a first dimension; and wherein thesequential application of the current pulses to the first plurality ofreset lines provides a larger current applied to the first plurality ofreset lines, for a given voltage, than if the current pulses had beensimultaneously applied to all the reset lines of the first plurality ofreset lines.
 11. The method of claim 10 further including: sequentiallyresetting group of magnetic sense elements of a second bridge circuit bysequentially applying current pulses to a second plurality of resetlines, wherein each reset line of the second plurality of reset lines islocated adjacent an associated group of magnetic sense elements of asecond bridge circuit, wherein each magnetic sense element of the groupof magnetic sense elements of the second bridge circuit senses themagnetic field in a second dimension; and wherein the sequentialapplication of the current pulses to the second plurality of reset linesprovides a larger current applied to the second plurality of resetlines, for a given voltage, than if the current pulses had beensimultaneously applied to all the reset lines of the second plurality ofreset lines.
 12. The method of claim 11 wherein the current pulses aresequentially applied to the first plurality of reset lines before thecurrent pulses are sequentially applied to the second plurality of resetlines.
 13. The method of claim 11 further including sequentiallyresetting group of magnetic sense elements of a third bridge circuit bysequentially applying current pulses to a third plurality of resetlines, wherein each reset line of the third plurality of reset lines islocated adjacent an associated group of magnetic sense elements of athird bridge circuit, wherein each magnetic sense element of the groupof magnetic sense elements of the third bridge circuit senses themagnetic field in a third dimension.
 14. The method of claim 12 wherein:the current pulses are sequentially applied to the first plurality ofreset lines before the current pulses are sequentially applied to thesecond plurality of reset lines, and the current pulses are sequentiallyapplied to the second plurality of reset lines before the current pulsesare sequentially applied to the third plurality of reset lines.
 15. Themethod of claim 10 further comprising: applying a stabilization currentadjacent first and second groups of magnetic sense elements of the firstbridge circuit; and measuring a field sensor response to the componentsof a magnetic field along a first direction.
 16. The method of claim 15further comprising: applying a stabilization current adjacent first andsecond groups of magnetic sense elements of the second bridge circuit;and measuring a magnetic field sensor response to the components of amagnetic field along a second direction.
 17. The method of claim 10wherein the applying the plurality of current pulses comprise applyingcurrent pulses having a pulse width of 50 nanoseconds or less.
 18. Amagnetic field sensor comprising: a first bridge circuit including afirst plurality of magnetic sense elements configured to sense amagnetic field in a first dimension, the first plurality of magneticsense elements organized i groups of magnetic sense elements where i isgreater than or equal to 2; a first group of reset lines, wherein eachreset line of the first group is located adjacent an associated group ofthe i groups of magnetic sense elements; a second bridge circuitincluding a second plurality of magnetic sense elements configured tosense a magnetic field in a second dimension, the second plurality ofmagnetic sense elements organized j groups of magnetic sense elementswhere j is greater than or equal to 2; a second group of reset lines,wherein each reset line of the second group is located adjacent anassociated group of the j groups of magnetic sense elements; a thirdbridge circuit including a third plurality of magnetic sense elementsconfigured to sense a magnetic field in a third dimension, the thirdplurality of magnetic sense elements organized k groups of magneticsense elements where k is greater than or equal to 2; a third group ofreset lines, wherein each reset line of the third group is locatedadjacent an associated group of the k groups of magnetic sense elements;and current driver circuitry configured to responsively and sequentiallygenerate a plurality of current pulses and sequentially apply thecurrent pulses to the reset lines of the first, second and third groupsof reset lines, wherein the sequential application of current pulses tothe reset lines results in a larger current applied adjacent to each ofthe i, j and k groups of magnetic sense elements, for a given voltage,than if the current pulses had been applied adjacent to the i, j and kgroups of magnetic sense elements simultaneously.
 19. The magnetic fieldsensor of claim 18 further comprising a logic circuit to generate thecontrol signals: one or more a contact pads configured to receive thecurrent pulses; wherein the first, second and third bridge circuits arelocated on a first chip and the logic circuit and current drivercircuitry are located on a second chip.
 20. The magnetic field sensor ofclaim 18 wherein each of the plurality of current pulses includes apulse width of 50 nanoseconds or less.